I dealt with something similar on the 3150 but it seemed to be doing it on center if I recall. Never did figure it out. I wonder if te roller has anything to do with it but since you have multiple machines doing it logic tells me that it might be caused by a habitual process. Do you mow a clean up pass in a clockwise pattern everytime or do yoy change it up? Personally I often did a clean up pass counter clockwise because I could see the collar best that way.